428 replies to this topic

[Sarkikos]

I observe in areas that are prone to prolonged heavy dew/frost.  If there is no dew/frost, that would be the exception.  Unless I put the dew shield on the scope, and wrap warming strips around the OTA behind the corrector, around the eyepiece, on the Telrad and around the finderscope objective and eyepiece, turn on the batteries and keep them on, dew/frost will form on the optics, guaranteed.  It's not really a question of if dew/frost will form, but when it will form:  sooner or later.  The warming strips DO prevent dew/frost.

 

So if I wrap the OTA in Reflectix, will I still need the warming strips to prevent dew/frost?  If there is a choice between no thermals and no dew/frost, I'd rather do without the dew/frost.  The effect of dew/frost on the corrector is a lot worse for observing than thermals in the tube.

 

Mike

Edited by Sarkikos, 26 November 2018 - 09:55 AM.

[Sarkikos]

Maybe it's just me, but I dislike the bright surface of Reflectix.  I really don't want that in my face when I'm observing at dark sites or at home in suburbia.

 

Also, at home in suburbia, I wouldn't want the bright glare from the Reflectix bringing attention to myself.  I don't even like observing with white OTA's for that reason.  I have no yard and have to observe in public areas in my community. 

 

Is there a good way to cover up the bright outer surface of the Reflectix?  Would doing that affect its effectiveness?

 

Mike

[Sarkikos]

Are there different types of Reflectix?  If so, which would be best for thermal control in Cats?

 

Mike

[petert913]

I don't understand how this material works.  Is it supposed to keep the air inside your SCT warmer - or slower to equalize?  I thought the whole

trick was to get our OTA's to cool to ambient temp quickly.    The reflective material doesn't provide any warmth on it's own so

what is it's real purpose? 

[Sarkikos]

I'm not an expert on Reflectix or the physics of thermals in Cats.  That's why I'm asking questions.  

 

My understanding is that Reflectix is wrapped around the OTA while it's still in the warm house before the OTA is taken outside to retain the heat that's in the OTA.  This is supposed to prevent thermals inside the OTA while we're observing. 

 

But if the Reflectix won't prevent dew/frost as well as warming strips, it's not for me, unless maybe it's possible to use both.

 

Mike

Edited by Sarkikos, 26 November 2018 - 10:09 AM.

[Dynan]

Flat black oil based paint will take the shine off of Reflectix. I use it as my dew shield, with black felt adhered to the inside to stop reflection.

 

Wrapping the OTA with Reflectix will slow thermal exchange. It is fast thermal exchange that causes air currents in the OTA. It has no heating properties in and of itself. I wrap a dew strip around the exterior of my SCT corrector to provide heat and prevent dew...which here in the deep South is prolific. The Reflectix goes around the the outside of my dew strip.

[Sarkikos]

I've heard that the reflective nature of the Reflectix is a major factor in why it works to retain heat.  It works by reflecting heat.  I can understand how reflecting heat coming up from the OTA back into the OTA would make sense. 

 

But why would reflecting away heat coming onto the OTA from the environment - the sky, the grass, the trees, whatever -  help the OTA retain heat?  That doesn't make sense to me.  So why does the outer surface of the Reflectix need to be shiny?  Why can't we just blacken it with paint?  Why would that affect the Reflectix' ability to help the OTA retain heat?  It's the inner surfaces of the Reflectix that face the OTA which are important as heat retaining surfaces.  The OTA never "sees" the outer surface of the Reflectix.

 

Mike

Edited by Sarkikos, 26 November 2018 - 12:08 PM.

[Sarkikos]

Yes, if I were using Reflectix as a dew shield, I would definitely flock the inside.  Why have a shiny, reflective surface right above the corrector?  A dew shield should also be a light shield. 

 

But then would the Reflectix still function fully to retain heat in the OTA if the inner surface of the Reflectix were covered by flocking?  Maybe it wouldn't matter, since we are talking about retaining heat - infrared - not visible light.  Whatever heat passes through the flocking will be reflected back by the Reflectix.  As long as the Reflectix reflects the heat back onto the OTA, it's functioning as required.

 

Mike

Edited by Sarkikos, 26 November 2018 - 12:06 PM.

[Sarkikos]

I wrap a dew strip around the exterior of my SCT corrector to provide heat and prevent dew...which here in the deep South is prolific. The Reflectix goes around the the outside of my dew strip.

Yep, that makes sense to me.  The Reflectix covering the dew strip will help retain heat coming from the dew strip, instead of having it radiate out into space.  The Reflectix should make the dew strip more efficient, have it last longer with less power used.

 

Mike

[555aaa]

1) I  have a "dew shield" made from reflectrix that I sometimes use, but I sewed black fabric to the inside to eliminate the shiny surface. Often it is too windy to use it.

2) In winter, I use a "scope cozy" on my 16" SCT plus a dew heater. The scope cosy is made of insulating drapery material which is fabric on both sides, plus insulation, plus an internal metal film layer. The dew heater is underneath the cosy. I leave about 4" of extra material to act as a dew shield.

3) The point of the insulation is to prevent heat transfer through the tube, and since there is no heat transfer, there are few thermal currents. But without the insulation, observing is hopeless and the dew strap at 5 amps can't keep up with the rate of dew formation.

4) The insulation does a great job of helping reduce the need to re-focus. 

[Sarkikos]

For smaller SCTs, would a Reflectix dew shield with a warming strip be sufficient to prevent tube thermals and dew/frost?  Or should the OTA be wrapped with Reflectix as well?

 

Mike

[Sarkikos]

I'm thinking that it doesn't matter if you paint the outer surface of the Reflectix or cover the inner surface with flocking.  As long as the Reflectix is present, it will retain heat coming from the OTA.  We're talking infrared vs visible light here, pretty much apples and oranges.  

 

For the dew shield made from Reflectix, you can flock the inside with ScopeStuff flocking.   And you can spray paint black the outer surface of Reflectix used for the dew shield or for the OTA.  It doesn't matter.  It won't affect the heat retaining capacity of the Reflectix.  So no shiny inner surface of the light shield.  No shiny outer surface of the Reflectix anywhere.

 

Am I wrong?  What am I missing here?

 

Mike

Edited by Sarkikos, 26 November 2018 - 01:02 PM.

[Dynan]

I've heard that the reflective nature of the Reflectix is a major factor in why it works to retain heat.  It works by reflecting heat.  I can understand how reflecting heat coming up from the OTA back into the OTA would make sense. 

 

But why would reflecting away heat coming onto the OTA from the environment - the sky, the grass, the trees, whatever -  help the OTA retain heat?  That doesn't make sense to me.  So why does the outer surface of the Reflectix need to be shiny?  Why can't we just blacken it with paint?  Why would that affect the Reflectix' ability to help the OTA retain heat?  It's the inner surfaces of the Reflectix that face the OTA which are important as heat retaining surfaces.  The OTA never "sees" the outer surface of the Reflectix.

 

Mike

As 555aaa said, the intended purpose of the Reflectix is to retard the temperature CHANGES in the OTA.

 

The OTA will reach thermal equilibrium (as best it can with changing outdoor temps) and the Reflectrix will retard any changes. It is the CHANGE in temp that causes thermal currents.

 

My dew strip is on the outer ring that holds the corrector. Not much heat enters the OTA in that setup. It mostly creates a small pocket of warm air in the dew shield. Hopefully enough to prevent dew/frost formation. The black felt is only there to prevent reflections from the shiny Reflectix.

 

After reflecting on the recurring reflection of Reflectix my brain reflexively retires. I need more coffee...

[Dynan]

I'm thinking that it doesn't matter if you paint the outer surface of the Reflectix or cover the inner surface with flocking.  As long as the Reflectix is present, it will retain heat coming from the OTA.  We're talking infrared vs visible light here, pretty much apples and oranges.  

 

For the dew shield made from Reflectix, you can flock the inside with ScopeStuff flocking.   And you can spray paint black the outer surface of Reflectix used for the dew shield or for the OTA.  It doesn't matter.  If won't affect the heat retaining capacity of the Reflectix.  So no shiny inner surface of the light shield.  No shiny outer surface of the Reflectix anywhere.

 

Am I wrong?  What am I missing here?

 

Mike

You are correct, Sir.

[Sarkikos]

I thought so.  But you wouldn't think so reading some posts in some threads on this topic.  

 

Mike

[Dynan]

Just for clarification: Reflectix is essentially 'bubble wrap' coated with 'mylar'.

The bubble wrap insulates.

The mylar reflects heat.

 

Put these both together and you have a layer of material that retards heat transfer, slowing thermal currents inside an OTA to prevent aberration of the photons we are trying to collect on sensors effectively the size of needle points. Subtle stuff.

[Sarkikos]

Just for clarification: Reflectix is essentially 'bubble wrap' coated with 'mylar'.

The bubble wrap insulates.

The mylar reflects heat.

 

Put these both together and you have a layer of material that retards heat transfer, slowing thermal currents inside an OTA to prevent aberration of the photons we are trying to collect on sensors effectively the size of needle points. Subtle stuff.

Yes, so it only matters that the Reflectix is wrapped around the OTA, to reflect back the heat coming from the OTA.  It doesn't matter if the outer surface of the Reflectix is painted black to prevent light glare.  It doesn't matter if the inner surface of a Reflectix dew shield is covered with black flocking to deaden ambient light.

 

Mike

Edited by Sarkikos, 26 November 2018 - 01:34 PM.

[WadeH237]

I've been using a Reflectix wrap on my C14 since this spring.

 

My setup is a single layer that is wrapped around most of the OTA.  The dovetail is exposed and the back of the telescope is exposed.  The Reflectix wrap is significantly longer than the OTA, so it extends well past the corrector plate.  Before wrapping the scope, I attach an AstroZap flexible dew shield which is about 1/2 longer than the Reflectix wrap.  So I have no bare reflectix exposed to the optics on the inside, so it cannot produce reflections visible in the eyepiece.  I also install a dew strap on the corrector end of the OTA, just behind the retaining ring that holds the corrector in place.

 

My usage scenario is that I travel to dark sky sites where I stay for multiple nights.  I set up the scope on the first day and leave it set up until I leave the site.  This can be anywhere from 4 nights to 2 weeks.  I leave the scope covered during the day with a Telegizmos 365 cover.  At dusk, once there is no direct sunlight hitting the OTA, I remove the cover, attach the dew shield and wrap the scope.  I don't bother to cover the scope when I am done observing at night.  I remove the wrap and the dewshield, and cover the scope, the next morning.  This means that my scope tends to be near ambient temperature most of the time.  I've not had any cases where I bring a scope from warm indoors to cold outdoors.  There have, however, been a few cases where I've uncovered the scope before sunset, and the dark grey OTA heats very quickly when exposed to any sunlight.

 

My experience has been that there is a dramatic improvement in the scope's performance in the early evening.  This past summer, we've had a nice display of planets early in the evening.  I am highly confident that the Reflectix wrap significantly improved my planetary observing, versus not insulating the scope at all.  The C14 mirror retains so much heat, that without some kind of mitigation for tube currents, it doesn't really settle down until many hours after sunset.  In cases where the temperature drops all night, it may never settle down at all.  This has not been a problem with the insulation.  I have also noticed that dew is significantly reduced, but in some cases, I've still needed some heat to keep the corrector clear.  (A C14 is among the world's best moisture collectors; we could potentially convert deserts to jungles just by setting up a bunch of C14s...)

 

As for visible light reflections, I don't have any Reflectix exposed to the scope's optics, as I mentioned above.  And since I observe at dark sky sites, there has not been any problems with reflections off the external, exposed Reflectix.  I have to say, however, that I don't think that covering or painting the exterior of the Reflectix would hurt it's performance at all in this application (but I've never tested it to be sure).  We're trying to keep heat from radiating out of the aluminum OTA, and there is no significant source of radiant heat trying to enter the scope at night.  I would say, though, that if you painted the Reflectix flat black and then exposed the scope to direct sunlight, you would probably make it heat up quicker than being unwrapped.

 

Those are my thoughts and experiences.  My suggestion is that you try it for yourself.  Reflectix is really cheap, and it's easy to wrap the scope.  There's no significant cost in trying different variations to see what works best for you.

[starman876]

interesting thread.  We all are learning a lot about thermal currents inside an SCT.  However, considering the scope wrapped up in insulation would keep internals of the scope at a certain temperature over outside ambient temp.  Would that not create some sort of heat thermal on the corrector no matter how long the dewshield?  

[Skywatchr]

I observe in areas that are prone to prolonged heavy dew/frost.  If there is no dew/frost, that would be the exception.  Unless I put the dew shield on the scope, and wrap warming strips around the OTA behind the corrector, around the eyepiece, on the Telrad and around the finderscope objective and eyepiece, turn on the batteries and keep them on, dew/frost will form on the optics, guaranteed.  It's not really a question of if dew/frost will form, but when it will form:  sooner or later.  The warming strips DO prevent dew/frost.

 

So if I wrap the OTA in Reflectix, will I still need the warming strips to prevent dew/frost?  If there is a choice between no thermals and no dew/frost, I'd rather do without the dew/frost.  The effect of dew/frost on the corrector is a lot worse for observing than thermals in the tube.

 

Mike

By all means, wrap the OTA!  The reflectix will help maintain the heat in the OTA and virtually eliminate tube currents.  And put your corrector dew strip in under the reflectix.  With the Reflectix your OTA will retain heat longer, and will also keep the corrector above ambient so no dew will form for a long time.  Once your OTA finally reaches ambient, you will probably need to power on the dew strip.  But you will enjoy a much longer observing time before needing to do so.  As far as your eyepieces and Telrad go, they are much smaller and won't retain heat.  You will still need the dew heaters for them.

[Dynan]

interesting thread.  We all are learning a lot about thermal currents inside an SCT.  However, considering the scope wrapped up in insulation would keep internals of the scope at a certain temperature over outside ambient temp.  Would that not create some sort of heat thermal on the corrector no matter how long the dewshield?  

There will ALWAYS be thermal exchange. The purpose of the Reflectix is to slow it down. It is rapid, wide-spread heating and cooling that causes thermal currents.

 

 

These are examples of what Reflectix will slow down, but never remove. The slower the current, the less aberration of the light.

 

Edited by Dynan, 26 November 2018 - 04:00 PM.

[WadeH237]

interesting thread.  We all are learning a lot about thermal currents inside an SCT.  However, considering the scope wrapped up in insulation would keep internals of the scope at a certain temperature over outside ambient temp.  Would that not create some sort of heat thermal on the corrector no matter how long the dewshield?  

I think that there are always heat thermals.  Even if the scope were completely acclimated, the observer at the eyepiece generates heat thermals that potentially impact the views.

 

I was reading a thread on another user group the other day where someone noted that his CCD camera was generating thermals that affected the image when the scope was pointed in certain directions.

 

I'll be the first to admit that I am not an expert in this area, but I have some armchair thoughts.  I think that SCTs have a particular sensitivity to thermals that are inside the baffle tube.  I've heard that if you are seeing a heat plume in a defocused star, then it's likely a current coming out of the inside end of the baffle tube.  Such a thermal is probably far more damaging to views than something near the corrector.

 

Going further, when I think about it, the corrector is a very low power lens.  A thermal there would be less impacting than a thermal between the primary and the secondary mirror because of the shape of the light cone (and I'd guess that closer to the secondary would be worse than closer to the primary).  The baffle would be worse still both because it channels thermals into a low area, and the most concentrated part of the light cone passes through the baffle.

 

I'm sure that there are people who could map out exactly what's happening inside the telescope - and it might be very different than I'd expect.  What I know from just trying things out, is what I related above.  For me, the Reflectix wrap makes my telescope better in the earlier parts of the night, and does not seem to harm it in the later parts of the night.  There might be an even better solution, but until I find it, I'll use what I've got.

[luxo II]

Would that not create some sort of heat thermal on the corrector no matter how long the dewshield?

If you do something really drastic you can indeed produce external thermal effects - eg by heating the corrector excessively with a hairdryer or worse, light a gas ring under the scope and point the OTA down to fill it with hot gas (yes I have seen SCT owners do that, in desperation to clear heavy dew). However these effects subside pretty quickly -10-15 minutes - as the surface of the OTA will cool quickly.

But simply insulating OTA the OTA has no adverse effects.

Try it, you will be surprised. I do this on my big maks and other SCT users here consider it normal, most have cannibalised either a yoga mat or an automotive reflective sun shade.

At home where the scope is stored the temps are typically 20 degrees C and the observing site drops to about -5 C so that’s roughly a 30 degree drop in F.

Edited by luxo II, 26 November 2018 - 04:36 PM.

[punk35]

In my case with my CPC11, I wrapped the whole tube including the mirror cell, and also made a dew shield painted flat black on the inside, that slides all the way down to the forks and extends about 14 inches past the corrector. 

 

Unlike WadeH, I usually only have time for short observing sessions. Before I wrapped the tube with Reflectix, I would follow the conventional practice of putting the scope out for two hours to acclimate. That works, but I live on the bad side of town and hated having to constantly keep an eye on the scope for hours before I could view, which is basically sitting on the sidewalk for everyone to see.  

 

Now, with the tube wrapped, I can pretty much just decide on a whim to take it out to view without giant heat plumes to deal with. I start viewing a soon as I get set up and the views are quite stable, barring atmospheric turbulence.  It’s almost like having a grab and go scope for quick views but it’s with a C11. Insulating the scope has given me more opportunities to view in short sessions than I had before. 

 

IMO insulating the scope has made for more stable views quickly, thereby increasing my observing time when I only have an hour or two. 

 

I plan to do more testing as the temps drop through the winter.  I’m curious to see how well it works going from 70f to 20f and below. It often seems like I get the best viewing conditions in the winter and that’s also when I have the most opportunities to view. We shall see. 

I also don’t have a dew strap or conventional dew shield, so it’s all up to the Relectix wrap and shield I made. 

[punk35]

I also agree that even with an insulated tube there must be heat exchange, but it seems like it is so gradual and gentle that we just don’t see it like we would notice strong plumes stirring up the air inside the scope. Just speculating but that’s all I got lol.